Coiled tubing operations on floating offshore drilling platforms typically require a motion compensated jacking frame. A motion compensated jacking frame supports a coiled tubing injector during a coiled tubing operation and includes a motion compensation system to offset potentially damaging relative motion between the coiled tubing injector, which is fixed to the platform, and the wellhead, which is fixed to the seafloor. As such, waves, currents and other external forces can cause damaging relative movements between the coiled tubing injector and the wellhead. Motion compensation systems protect the wellhead by reducing the load transferred from the coiled tubing injector to the wellhead when such relative motions occur.
However, during a coiled tubing operation the coiled tubing may become “hung up” or “stuck” in the well for many various reasons. This results in the transfer of a large load or a rapid load change on the wellhead as the coiled tubing injector is pulled down onto the wellhead in reaction to the “sticking” of the coiled tubing string in the well. Conventional passive coiled tubing motion compensation systems do not have the capability to react to such large rapid load changes. Instead, such systems are designed to detect relative motions between the coiled tubing injector and the wellhead and react by applying a relatively constant accumulator spring rate in response thereto.
Passive compensation systems provide some degree of force isolation between the stationary and moving components by use of what can be analogized to a low frequency dampener. Passive compensation systems typically include cylinders in combination with a compressible fluid circuit, such as nitrogen bladder hydraulic accumulators or direct air pressurization, to allow cylinder displacement with only moderate variation in applied force.
When applied to coiled tubing operations, the coiled tubing injector is placed on a structure that is supported by the aforementioned cylinders on one end and connected to a structure anchored to the floating platform on the other end. When a relative motion occurs between the wellhead and the platform (and hence the coiled tubing injector supported on the platform), the cylinders displace resulting in a moderate change in the force of the supported load on the injector (and hence the load on the wellhead.)
Typically, nitrogen charged accumulators are connected to the cylinders to provide a relatively constant hydraulic pressure to the cylinders. The accumulator nitrogen charge pressure is adjusted to maintain a relatively constant force throughout the cylinder stroke. Such an arrangement is adequate when the load on the wellhead remains constant, however, in most coiled tubing operations the load on the wellhead varies anytime the coiled tubing string weight is increased or decreased due to the addition or removal of coiled tubing from the well.
As such, operator adjustment of the cylinder stiffness coefficient is typically required to account for this dynamic loading of the wellhead. That is, when the load on the wellhead is increased, the cylinder stiffness coefficient may be manually increased by adding nitrogen support bottles to the system to increase the nitrogen pre-charge in the accumulators; and when the load on the wellhead is decreased, the cylinder stiffness coefficient may be manually decreased by bleeding off some of the nitrogen pre-charge pressure to maintain a desired compensating force. Accordingly, a need exists for an improved compensation system for a jacking frame.